The present invention relates to a process for hardening elongate metal elements, in particular solid and hollow circular section rectilinear bars of great length, and including both those with a variable section and those with a constant section.
In general, hardening of a metal bar of the type described above, that is to say a bar having relatively high length/diameter ratio, involves numerous problems both during the heating phase and during the cooling phase which constitutes the hardening phase itself. It is, in fact, clear that a non homogeneous heating of such a bar and, above all, non homogeneous cooling thereof gives rise to internal stresses which are manifested automatically as deformations of the axis of the bar.
In general the heating of the bar of the type described above is performed by utilising vertical furnaces from the top of which the bars, once having reached the hardening temperature, are withdrawn by means of a tower structure and immersed into a cooling tank containing water.
The process described above has been found to be unsuitable for hardening bars of relatively great length in that the heating in the vertical furnace makes it difficult to obtain a uniform temperature along the whole of the bar. Moreover, hardening by immersion in water normally induces in the bars an irregular distribution of stresses due to the formation of cushions of steam constituting an insulating layer.
Because of the above explained disadvantages the bars, once hardened, are normally distorted. In particular, originally rectilinear bars are normally twisted and bent and must be subjected to expensive "reformation" processes by plastic deformation to be brought back to their original rectilinear configuration. The "reformation" of the hardened bars can in part be eliminated in cases in which the bars themselves must be subjected to mechanical working subsequent to hardening. In this case, a relatively thick layer of removable stock can permit the compensation of distortions, but involves a significant waste of material.